Efficient bandwidth utilization methods for catv docsis channels and other applications

ABSTRACT

Methods to improve the data carrying capacity of CATV DOCSIS systems and other communications systems are disclosed. Communications channels may be more efficiently spaced with reduced or absent guard bands by using receivers with adaptive signal cancellation methods, equalizing circuits, or polyphase filter banks and Fast Fourier Transform signal processing methods to correct for higher levels of cross-talk. QAM type communications channels may also be utilized on a synchronized two-transmitter at a time basis by adjusting the transmitters to predefined signal levels, such as +1, −1, +½, −½ to enable the combined signals to be distinguished at the receiver. These two methods may be combined to create a still higher data throughput system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of U.S. provisionalapplication 61/622,132, entitled “EFFICIENT BANDWIDTH UTILIZATIONMETHODS FOR CATV DOCSIS CHANNELS AND OTHER APPLICATIONS”, inventorShlomo Selim Rakib, filed Apr. 10, 2012; the contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

This application is in the field of wired and wireless communicationsmethods.

DESCRIPTION OF THE RELATED ART

The Data Over Cable Service Interface Specification (DOCSIS) series ofstandards and protocols for transmitting large amounts of data overCable TV systems is now in widespread use. This method, originallyintroduced in 1997 by CableLabs, enables cable TV systems, originallyintroduced in the 1940's and 1950's pre-Internet era for the purpose ofcarrying analog television signals to the home, to be re-purposed andextended to now have many additional Internet and digital communicationsuses as well.

In its current DOCSIS 3.0 specification, the DOCSIS standard now allowsvarious CATV households to use their respective cable modems to receivedownstream digital data from the CATV cable head and cable plant, aswell as use their cable modems to transmit digital data back upstreamfrom the various households back to the cable plant and cable head.

As the popularity of modern DOCSIS CATV methods have grown, and as moreand more home users become accustomed to transmitting large amounts ofdata (e.g. though high definition webcams for video conferencing) aswell as receiving large amounts of data, an increasing number of CATVusers are bumping up against the limits of present CATV datatransmission methods. The problem is particularly acute for upstreamsignals, because many household modem users must share a comparativelysmall amount of upstream CATV bandwidth.

As a result, there is a considerable amount of commercial interest infinding ways to further extend the data carrying capacity of presentDOCSIS CATV protocols, systems, and methods, particularly with regardsto upstream data transmissions.

Under present DOCSIS protocols and methods, upstream data transmissionfrom the various household cable modems is confined to the 5 MHz to 42MHz frequency regions (in the US). Under the DOCSIS requirements thesevarious cable modems contain software frequency adjustable transmittersthat can assign the modems various time and frequency slots fortransmitting. However due to the necessity of maintaining adequateseparation between DOCSIS channels, a fair amount of this limited 5-42MHz upstream region is presently occupied by empty guard bandfrequencies. Under prior art methods, these guard band frequencies areneeded to minimize cross-talk between adjacent DOCSIS channels, butotherwise are undesirable because they consume valuable bandwidth.

Under the DOCSIS protocols, each DOCSIS channel is transmitted usingQuadrature Amplitude Modulation (QAM) modulated waveforms. Thesewaveforms transmit symbols (usually between about 2-8 bits per symbol)by varying the phase angle and intensity of the QAM waveforms. Forexample, for downstream transmission, the DOCSIS 3.0 specificationutilizes 64 level or 256 level QAM modulated waveforms. By contrast, forupstream data, the DOCSIS 3.0 specification generally utilizes 4, 8, 16,32, or 64 level QAM signals. This translates into 2, 3, 4, 5, and 6 bitsper QAM symbol respectively.

BRIEF SUMMARY OF THE INVENTION

Due to the historical development of the various DOCSIS specification,the technological constraints operating at the time tended to enforcethe concept that there must be comparatively large guard bands betweendifferent DOCSIS frequency channels. Although an acceptable solution atthe time, today's data needs now make it less and less acceptable toallocate scarce bandwidth to maintain such prior art guard bands.

The invention is based, in part, on the insight that recent advances insignal processing technology now make it both feasible and costeffective to employ more sophisticated signal processing schemes toreduce or eliminate such relatively large guard bands.

The invention is further based, in part, on the insight that due to theenormous capital investment in DOCSIS compliant systems, tens ofmillions or more of household cable modems and associated supportequipment, methods that improve the efficiency of DOCSIS datatransmission that are generally backward compatible with this hugecapital investment in DOCSIS systems are of high commercial value.

The invention is further based, in part, on the insight that the tens ofmillions of household cable modems presently in use under the DOCSIS 2.0and 3.0 specifications may have additional flexibility capability thatthat is not being fully utilized. In particular, due to the programmablenature of modern DOCSIS cable modem transmitters, these in-place DOCSIScable modems can, in principle, if improved CATV cable plant and headside DOCSIS receivers are provided, be simply upgraded to transmit moredata upstream by the transmission of the proper set of DOCSIS cablemodem software commands.

The invention is further based, in part, on the insight that for thesepurposes, it is desirable to provide improved CATV cable plant and headside DOCSIS receivers that employ modern techniques in adaptive signalcancellation/signal separation and equalization technology. When suchimproved CATV cable plant and head side DOCSIS receivers are provided,and the proper transmitter setting commands are sent to even prior artDOCSIS cable modems, such as DOCSIS 3.0 cable modems, it becomesfeasible to improve the data carrying capacity of DOCSIS downstream datatransmissions at a relatively low cost, and on a relatively rapidimplementation schedule. Here for example, once DOCSIS cable modemsetting commands are transmitted to the various cable modems in a CATVneighborhood, these now reset cable modems can now transmit upstreamusing a greater number of now more closely spaced adjacent upstreamchannels. The improved DOCSIS receivers can in turn correct forcross-talk between these adjacent channels using such improved adaptivecancellation/signal separation/equalization methods.

The invention is also based, in part, on the insight that in addition tothe previously discussed methodology, there is also a different, secondmethodology to improve DOCSIS upstream data carrying capacity as well.This second methodology is also based on providing improved CATV cablehead and plant side DOCSIS receivers, and it also operates bytransmitting an alternate set of CATV cable modem upstream transmittercommands to prior art cable modems, such as DOCSIS 3.0 cable modems.

In this different second methodology is based, in part, on the insightthat changes in various DOCSIS QAM modulation schemes, in particular onthe DOCSIS 3.0 upstream 8, 16, 32, or 64 level QAM data modulationmethods, can also be made at the cable modem side by issuing variousDOCSIS cable modem software commands. If improved DOCSIS cable plant orcable head side receivers are provided, it is possible to reconfigureprior art DOCSIS cable modems to now allow two different cable modems totransmit on the same upstream channel frequency, and at the same time,effectively providing a low-cost and rapidly implemented scheme tonearly double upstream data transmission rates.

This second methodology relies, in part, on the synchronous nature ofprior art DOCSIS protocols, such as the DOCSIS 3.0 protocols. Accordingto this aspect of the invention, two cable modems, for example, may beprogrammed or otherwise set to transmit upstream on the same channel atthe same time. However to enable an improved DOCSIS cable plant or cablehead end receiver to distinguish the two different cable modems, onecable modem, for example may be set to transmit with full intensity(e.g. transmit pluses of levels +1 and −1). By contrast, and again as anexample, the other cable modem may be set to transmit with half (e.g. ½)intensity (e.g. transmit pluses of levels +½ and −½). As will bediscussed in more detail in the QAM constellation discussion, theproperties of QAM signals inherently provide sufficient information thatan improved DOCSIS receiver can distinguish between the two differentcable modem transmitters.

Thus, for example, an improved cable plant or cable head DOCSIS receivercapable of discriminating between the various combined signal levels(e.g. 1½, ½, −½, and −1½) may be used for this purpose.

The two different but related improvements: more channels throughnarrower channel separation, and transmitting two cable modems at a timethrough alternate QAM modulation methods, are each capable of operatingindependently. However in a preferred embodiment, because the twodifferent methods both generally require upgraded DOCSIS cable plant orhead end receivers, and because the two different methods both alsooperate by sending alternate types of cable modem transmitter adjustmentcommands to the various household cable modems on a particularneighborhood stretch of CATV cable, in a preferred embodiment, bothmethods may be enacted at the same time and in the same upgrade package.

In combination, the two methods, systems, and devices described hereincan thus combine to create a synergistic improvement in DOCSIS datacarrying capacity. Although throughout this specification, the exampleof use of these two methods for CATV DOCSIS applications, andparticularly for upstream CATV DOCSIS signal carrying applications isused as a specific example, it should be understood that the variousapplications of the invention's methods apply to other types of bothwire and wireless data transmission as well.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of how the invention's Nyquist spacedtransmitters can more densely pack multiple communications channels intoavailable bandwidth.

FIG. 2A shows an embodiment of a Nyquist spaced DOCSIS receiver

FIG. 2B shows more details of a feedback structure that can be used forcorrect for crosstalk between closely spaced channels.

FIG. 2C shows a decision feedback structure that can be used to correctfor crosstalk between closely spaced channels.

FIG. 3 shows an example of two users (two cable modem transmitters)transmitting upstream on the same channel and at the same time, oneusing a QAM16 transmission scheme at a first signal intensity, and theother using a QAM16 transmission scheme at a second and lower intensitylevel (symmetric scheme).

FIG. 4 shows an example of the how the DOCSIS receivers can receive thetwo combined high intensity and low intensity QAM-16 signals as aQAM-256 signal, and then determine which symbols came from whichtransmitter.

FIG. 5 shows an example where the first user is transmitting using ahigh intensity QAM-64 signal, and the other user is transmitting using alow intensity QAM-4 signal (asymmetric example).

FIG. 6 shows how the DOCSIS receivers can receive the combined highintensity QAM-64 and low intensity QAM-4 signals and again determinewhich symbols came from which transmitter.

FIG. 7 shows another asymmetric example where the first user istransmitting a high intensity QAM-4 signal, and the other user istransmitting using a low intensity QAM-64 signal.

FIG. 8 shows how the DOCSIS receivers can receive the combined highintensity QAM-4 signal and the low intensity QAM-64 signal and againdetermine which symbols came from which transmitter.

FIG. 9 shows a diagram of how the invention's techniques of crosstalkcorrection and using two cable modem QAM transmitters on one channel maybe combined to improve the upstream data carrying capability of a CATVcable system.

DETAILED DESCRIPTION OF THE INVENTION

Terminology: In modern telecommunications technology, communicationssignals are often transmitted as various digitally modulated waveforms.Typically the maximum symbol rate (symbols per second) of acommunications channel is equal to a constant of proportionality “B”times the bandwidth of a channel (in Hz).

Here the maximum symbol rate is often denoted as Fs.

Thus Fs(symbols/second)=B*Bandwidth(in Hz).

If each symbol in turn carries “n” bits, then the bitrate of acommunications channel, in bits per second is thus the same constant “B”times the number of bits per symbol “n” times the bandwidth of thechannel.

Bitrate(bits/second)=n*Fs=n*B*Bandwidth(in Hz).

Conversely, the spectral efficiency of a channel is often denoted asbitrate/bandwidth, or essentially the constant “B” times the number ofbits per symbol.

Spectral efficiency=Bitrate/Bandwidth=B*n

In communications technology discussions, often various communicationsdata parameters are expressed in units of bits per second, and Hz. As aresult, the constant of proportionality “B” is often a number equal toor close to 1. When “B: is 1 or close to 1, at least to a firstapproximation, the maximum symbol rate (symbols per second) of acommunications channel is equal to the bandwidth of the channel (in Hz).

Fs(symbols/second)≈Bandwidth(Hz).

Standard terminology in communication engineering is a bit informal inthis regard. Thus often communications engineers use the terms “symbolrate” (Fs) and bandwidth interchangeably, even though the units are infact somewhat different (e.g. symbols per second vs. spectral spread inHz). Although this specification will occasionally follow this standardcommunication engineering usage, and terminology conventions, it shouldbe understood that those skilled in the field will understand thatsymbol rate and bandwidth are in fact related by a constant ofproportionality “B”. B may be a conversion factor with value 1, or aconversion factor with a value that is often less than 2, such as 1.8.

Roll-Off Rate Discussion:

In order to efficiently transmit signals in the form of digitallymodulated waveforms, it is not feasible to transmit perfect square wavesor spikes, such as Dirac delta pulses. Rather, due to various real worldlimitations, all transmitted signals end up being spread somewhat overthe time domain by that particular communications channel's impulse orfrequency response parameters. To minimize intersymbol interference(ISI) due to this real-world spreading, communications engineers insteadtransmit various types of shaped waveforms optimized to minimize ISI.

In particular, often these various ISI optimized waveforms are shapedand transmitted according to a raised-cosine filter. The edges of suchraised-cosine filter type waveforms typically do not end abruptly, butrather roll off in a gradual manner. The shape and extent of thisroll-off spreading can be set to a value that is most appropriate forthat particular channel's time domain characteristics and frequencydomain characteristics. These considerations are often referred to as achannel's Nyquest ISI criterion.

The net effect of these various considerations is that in real worldcommunications channels, here again exemplified by DOCSIS CATV channels(particularly upstream channels), this signal roll-off can cause thesignals from one communications channel to interfere (cause cross talkwith) with the signals from another communications channel unless thechannels are separated by guard bands. As previously discussed, however,these guard bands waste valuable communications spectrum. However toavoid interference, prior art tended to regard them as a necessary evil.

First Aspect: Improved Guard Band Methods

In one embodiment, the invention may be viewed as a method of improvingthe upstream data transmission rate of a DOCSIS CATV cable system. Inthis embodiment, this method will generally comprise providing animproved DOCSIS receiver capable of correcting for cross-talk betweenadjacent DOCSIS channels. The invention also calls for directing aplurality of DOCSIS cable modems to transmit upstream using a pluralityof adjacent channel frequencies selected with reduced or absent guardbands.

For example, whereas the prior art DOCSIS guard band width would be onthe order of α*Fs, where Fs is the bandwidth of each said channels, anda is the roll-off factor of the bandwidth of each of said channels,according to the invention, this guard band might only be half of lessof this value.

Indeed, when a plurality of channels, such as N channels, are beingtransmitted on adjacent channels (e.g. closely spaced with no otherchannels are between them), then the bandwidth of these guard bands canbe reduced to zero or nearly zero. Thus neglecting edge effects, N suchadjacent channels would occupy N*Fs bandwidth. When including edgeeffects (i.e. the extreme two extreme boundaries of the N adjacentchannels), then according to the invention, the N such adjacent channelswould occupy (N+α)*Fs bandwidth.

Although normal DOCSIS cable modems transmit upstream with asubstantially larger frequency separation (guard bands) betweenchannels, even under the DOCSIS specification, the frequency and spacingof such transmitters is under software control. Thus the transmitterscan be directed to retransmit at a non-standard and substantially closerfrequency separation by transmitting various commands to these cablemodems (such as appropriate DOCSIS Offset Frequency Adjust parameters),to set the relevant cable modems to transmit on the desired, moreclosely spaced, frequencies.

The invention's improved DOCSIS receiver will generally receive thisplurality of adjacent channels over a plurality of channel frequencies;and correct for cross talk between these adjacent DOCSIS channels. Aswill be discussed, this can often be done by various adaptive signalcancellation/signal separation/signal equalization methods.

In particular, in some embodiments, such adaptive signal separationtechniques may operate by partitioning the incoming closely spacedDOCSIS channel signals through a polyphase filter bank, and furtherprocessing the output from said polyphase filter bank using Fast FourierTransform methods. This may be done through one or more Digital SignalProcessors or other electronic signal processing techniques.

Examples

As previously discussed, the DOCSIS specification calls for a guard bandbetween different channels. As shown in FIG. 1 top, usually the edge ofthese DOCSIS channels tail off in intensity as a function of frequencyaccording to a raised-cosine function (e.g. cos(θ)²) with a roll-offfactor α. To prevent crosstalk, there will thus generally be guard bandsbetween adjacent channels with at least a frequency spacing of α*Fs,where, as previously discussed Fs, the bandwidth of the channel (whenB=1), is proportional to the channel symbol transmission rate, and underthe Nyquist rule is also proportional to the sampling rate (when thewaveforms are sampled or digitized).

According to the invention, however, these guard bands of approximatebandwidth α*Fs can instead be reduced (e.g. to a substantially lowerbandwidth such as one half α*Fs or less) or eliminated, and the variouscommunication channels moved closer together.

If this is done, then for “N” communications channels, instead of the Ncommunications channels taking at least N*(1+α)*Fs of valuable spectrumbandwidth (which is the minimum prior art value), instead according tothe invention, now the N communications channels will take only (N+α)*Fsbandwidth. The excess spectrum bandwidth now released by this moreefficient packing of N communication channels can then be used for otherpurposes, such as for additional downstream communications channels. Thenet effect is to cram additional downstream communications channels intothe same bandwidth allocation, such as the 5-42 MHz (for the US) DOCSISupstream region.

Here, as previously discussed, prior art DOCSIS cable modems, such asDOCSIS 3.0 cable modems, are capable of receiving DOCSIS commands thatin turn program the cable modem transmitters to now to transmit withcloser frequency spacing. Thus the problem is not on the transmitterside, the problem is on the CATV cable or head end receiver side. If theguard bands between the various DOCSIS upstream channels are reduced,and if prior art DOCSIS receivers are used, then the received upstreamsignals will be degraded due to cross-talk effects.

According to one aspect of the invention, this crosstalk problem can beresolved by using improved CATV cable or head end DOCSIS receivers thatuse adaptive canceling techniques, or equalization techniques, forimproved signal separation to correct for this crosstalk.

One method to produce such improved receivers is to utilize conventionalreceivers, but also put in equalizer circuitry to correct forcross-talk.

Alternatively, in other embodiments, teaching similar to that of Harriset. al., “Digital Receivers and Transmitters Using Polyphase FilterBanks for Wireless Communications” IEEE Transactions on Microwave Theoryand Techniques Volume 51(4), 1395-1412, (2003) may be used.

In this alternative scheme, the DOCSIS receivers needed to receive thesemore densely packed waveforms can operate using a combination of FastFourier Transform (FFT) methods and Polyphase filter methods. Accordingto this Polyphase filter approach, the received frequency range isoversampled with a plurality of filters, and the received spectrum isthen divided into many regions, such as M regions. This method has theadvantage that it can be more easily implemented on modern and ofteninexpensive Digital Signal Processor (DSP) chips.

Other methods of cross talk correction are also possible. An example ofsuch an improved DOCSIS receiver, which uses equalization techniques tocorrect for crosstalk between adjacent communications channels, is shownin FIG. 2A.

FIG. 2B shows more details of a feedback structure that can be used forcorrect for crosstalk between closely spaced channels.

FIG. 2C shows a decision feedback structure that can be used to correctfor crosstalk between closely spaced channels.

Note that although CATV DOCSIS channels, and in particular upstreamchannels, are used as a specific example of this aspect of theinvention, this example is not intended to be limiting. This type ofapproach may also work for other applications, such as wireless cellphone channels, for example.

Thus from a commercial implementation perspective, by simply upgradingthe DOCSIS receivers at various neighborhood CATV junctions, such asoptical fiber junctions, and by programming the neighborhood cablemodems to transmit with narrower frequency spacing, then, for examplethe upstream capability of an existing DOCSIS installation can besubstantially upgraded at a relatively low cost.

Second Aspect: Packing More Bits into Each Communications ChannelThrough Fine Combined QAM Constellation Processing

Background: The DOCSIS 2.0 specification and above calls transmittersand receivers which can transmit and receive signals using either aSynchronous Code Division Multiple Access (SCDMA) mode; or an AdvancedTime Division Multiplexing (ATDMA) mode.

For the purposes of this invention, often use of the SCDMA mode ispreferable because this mode calls for accurate transmitter (e.g. cablemodem transmitter) synchronization, and this in turn makes receiverimplementation simpler for both aspects of the invention. That is SCDMAmode facilitates the adjacent channel crosstalk cancellation process,which is used in the first aspect of the invention. SCDMA mode alsofacilitates the fine combined QAM (quadrature amplitude modulation)constellation processing techniques, which are used in the second aspectof the invention. These fine combined QAM constellation processingtechniques are discussed in more detail below.

In particular, with regards to the fine combined QAM constellationprocessing second aspect of the invention, SCDMA mode further allows forgreater flexibility in defining the combined QAM constellation size percode and bit allocation among users, as will also be discussed in moredetail below.

In some embodiments, this second aspect of the invention may be a methodof further improving the upstream data transmission rate cable modems ona DOCSIS CATV cable system. Here as well, the invention will generallyalso provide an improved cable or head end DOCSIS receiver system. Inthis second embodiment, the improved cable or head end DOCSIS receiversystem will be capable of frequency synchronizing and transmitter leveladjusting a plurality of neighborhood DOCSIS cable modems, each with itsown upstream DOCSIS transmitter. DOCSIS Carrier and Clocksynchronization, as well as phase synchronization is also required.

As shown in more detail in FIG. 9, generally CATV cable systems comprisea plurality of cable modems (804-816), each attached to a neighborhoodsection of CATV cable (802), and each communicating to a cable head withDOCSIS receivers (800, 820), or an optical fiber junction (824) which inturn eventually communicates with DOCSIS receivers. For the purposes ofdiscussion of this aspect of the invention, however, let us focus onjust two of the many neighborhood cable modems, such as (804) and (806).In this case, assume that there are so many other neighborhood cablemodems that the upstream channels are fully utilized, and thus it isadvantageous to “overload” the prior DOCSIS system, which only allowedone cable modem transmitter to transmit at any given time and any givenchannel, by now allowing both a first cable modem and a second cablemodem to transmit at the same given time, and same given channel.

To do this, according to the invention, the cable head (820) or otherdevice (824) may, after appropriate carrier and clock synchronization,as well as phase synchronization, direct at least a first DOCSIS cablemodem (804) to upstream transmit its first signals at a first range ofsignal intensity levels at a given time and given frequency. Theinvention will also direct at least a second DOCSIS cable modem (806) totransmit its second signals at a second range of signal intensity levelsat the same time, same frequency, and same SCDMA code as the firstDOCSIS cable modem. Thus the system's improved DOCSIS receivers will endup receiving a combined first and second DOCSIS cable modem signals(852). As will be discussed, however, by suitable selection ofappropriate QAM transmission protocols, and using the invention'smethods, it is feasible to use this improved DOCSIS receiver to separatethe first signals from the second signals. The net effect is toessentially double upstream data carrying capacity because now two cablemodems may upstream transmit at the same time and using the samecommunications channels.

To do this, in one embodiment, the invention may direct the first DOCSIScable modem (804) to upstream transmit its first signals at a firstrange of signal intensity levels, and also direct the second DOCSIScable modem (806 _(—) to upstream transmit its second signals at asecond range of signal intensity levels. This can be done, for example,by transmitting the appropriate DOCSIS commands, such as at least oneDOCSIS Power Level Adjust or Transmit Equalization Set parameters to thefirst and second cable modems (804), (806). For example, the secondrange of signal intensity levels may be set to a value that is half thatof the first range of signal intensity levels.

Generally, according to this method, the first DOCSIS cable modem (804)may be directed to upstream transmit using the same type of QAM protocolas the second DOCSIS cable modem (806). Alternatively, howeverasymmetric transmission methods may be also used, wherein the firstDOCSIS cable modem may be directed to transmit using a type of QAMprotocol that is different from the second DOCSIS cable modem.

Examples

As previously discussed, upstream DOCSIS generally uses sends datamodulated according to an 8 (3 bits), 16 (4 bits), 32 (5 bits), or 64 (6bits) level QAM data modulation method. However, because DOCSIS usessynchronized transmission methods, it is possible to essentially combinethe relatively coarse (e.g. 4-bit QAM-16) transmissions from twodifferent users using two different cable modems into a combinedtransmission that will essentially transmit more bits of data per symbolper time slice.

With this scheme, an improved DOCSIS receiver, which in some embodimentsmay be a standard DOCSIS QAM receiver with software improvementsdesigned to separate the first cable modem transmitter from the secondcable modem transmitter, can then be used to separate out the twosignals from the two transmitters at the receiving end. Here this cablemodem transmitter separation method can be relatively simple, such as,for example separating out the least significant bits of the QAM signalfrom the most significant bits of the QAM signal, optionallysupplemented by additional algebraic or linear algebraic solutionmethods (e.g. for noise correction) as desired.

To do this, generally in addition to timing synchronization between thedifferent cable modems, there must also be frequency synchronization andtransmitter level (signal intensity) control as well. This method alsoassumes an adequate signal to noise level (SNR) at the receiver

FIG. 3 shows an example of two users (two cable modem transmitters)transmitting upstream on the same channel and at the same time, oneusing a QAM-16 transmission scheme at a first signal intensity, and theother using a QAM-16 transmission scheme at a second and lower intensitylevel (symmetric scheme). The QAM constellation values of the second andlower intensity level transmitter are shown underlined.

FIG. 4 shows an example of the how the DOCSIS receivers can receive thetwo combined high intensity and low intensity QAM-16 signals as aQAM-256 signal, and then determine which symbols came from whichtransmitter. Here the first and higher level transmitter shows up as thehigher level bits of the combined QAM signal, while the second and lowerlevel transmitter shows up as the lower level (underlined) bits of thecombined QAM signal. Thus with appropriate software, the improved DOCSISreceiver can easily separate out the high order bits (400) from the loworder bits (402) and determine the original signals from the twotransmitters.

Turning briefly to FIG. 9, the results shown in FIG. 3 can, for examplebe obtained from two different cable modems (804) and (806) eachtransmitting a QAM-16 signal from two users (i.e. a first user and asecond user) at the same time and same frequency. The resulting combinedsignal originally shown in FIG. 4, is seen by the cable head or plantreceiver (824), (820).

Note that FIGS. 3-8 generally follow the conventional QAM constellationdiagram convention, where the QAM “Q(t)” (Phase) signal is on the “Y”axis (300), and the QAM “I(t)” (Intensity) signal is on the “X” axis(302). The communication bits transmitted by the QAM constellation(s)are, in some figures, shown as the various shades of black and whiteinside the circles, which represent the various QAM constellationpoints. This QAM constellation scheme thus can distinguish betweendifferent QAM signals of different amplitude; this aspect is useful,because an improved QAM receiver following this type of QAMconstellation scheme can therefore distinguish between two differentsimultaneous QAM transmitters, as will be discussed.

Thus, in FIG. 3, as previously discussed, one user and one cable modemwill transmit at a higher (e.g. +1 and −1) level of signal intensity(the gray circles) while the other user and other cable modem willtransmit a lesser intensity level, such as a +½ and −½ level of signalintensity (the white circles shown in dotted box 304). The signalintensity levels of the lower intensity transmitter are shown underlinedon the “I” axis as (306), and on the “Q” axis as (308).

Note that this +1 and −1 example illustrates the case for a two leveltransmitter with symmetric bit per symbol allocation.

Note further that the particular transmit leveling scheme used in thisaspect of the invention generally depends on both the respective bit persymbol allocation, and also path loss. The leveling algorithm willcontrol each QAM transmitter to form the final desired QAM constellationwhich maximizes the bits per symbol, subject to available SNR to decodethe bits.

In an alternative “symmetric” scheme (i.e. each user and cable modem istransmitting using the same type of QAM protocol), a first user couldtransmit at 4 bits per symbol with grid points +/−4, 12; and the seconduser could transmit at 4 bits per symbol with grid points +/−1, 3. Inthis case, the two signals together again generate an 8-bit like QAM-256like signal with a relative transmit symbol spacing of 4:1.

Put alternatively, QAM-16 sends 4 bits per symbol. Here, the 4 bits fromthe first cable modem QAM transmitter and the 4 bits from the secondcable modem QAM transmitter combine to produce a (2[transmitter1]+2[transmitter 2]) bit combination on the QAM “I” axis, and a(2[transmitter 1]+2[transmitter 2]) bit combination on the QAM “Q” axis.Due to the differing signal intensity levels, the QAM signals from thetwo channels can be solved for (e.g. determined) by improved softwareoperating at the invention's improved QAM receiver(s).

By contrast, FIG. 5 shows a QAM constellation diagram of an asymmetricexample. Here the first user is transmitting using a higher intensityQAM-64 signal (gray circles), while the second user is transmittingusing a lower intensity QAM-4 signal (white circles, box 500). The lowerintensity signals are again shown underlined. The two signals togetherare received by the DOCSIS receiver as the combined QAM-256 signal shownin FIG. 6. The DOCSIS receiver can again determine which symbols camefrom which transmitter by, for example, separating out the high orderbits (not underlined) from the low order bits (underlined).

Here, QAM-64 transmits 6 bits per symbol, and QAM-4 transmits 2 bits persymbol. Here, the 6 bits from the first cable modem QAM transmitter andthe 2 bits from the second cable modem QAM transmitter combine toproduce a (3 [transmitter 1]+l[transmitter 2]) bit combination on theQAM “I” axis, and a (3 [transmitter 1]+1[transmitter 2]) bit combinationon the QAM “Q” axis. Here as well, due to the differing signal intensitylevels, the QAM signals from the two channels can be solved for (e.g.determined) by such high/low-order bit-separating (e.g. improved)software operating at the invention's improved QAM receiver(s).

In another “asymmetric” example, the two simultaneous users may betransmitting on two different cable modems by different QAM modulationmethods. For example, the first user may be transmitting at 2 bits persymbol using higher intensity QAM-4 modulation, (e.g. using +1, −1signal levels), while the second user may be transmitting at 6 bits persymbol using QAM-64 modulation and lower intensity modulation (e.g. +½and −½ signal levels). This alternative example is shown in FIG. 7.Here, the first user is transmitting a high intensity QAM-4 signal (graycircles), and the other user is transmitting using a low intensityQAM-64 signal (white circles, box 700). Here again the lower intensitysignals from the low intensity transmitter are shown underlined.

FIG. 8 shows how the DOCSIS receivers can receive the combined highintensity QAM-4 signals and the low intensity QAM-64 signals, separateout the bits according to high order bits and low order bits, and againdetermine which symbols came from which transmitter.

Here, QAM-4 transmits 2 bits per symbol, and QAM-64 transmits 6 bits persymbol. Once again, the 2 bits from the first cable modem QAMtransmitter and the 6 bits from the second cable modem QAM transmittercombine to produce a (1[transmitter 1]+3 [transmitter 2]) bitcombination on the QAM “I” axis, and a (1[transmitter 1]+3 [transmitter2]) bit combination on the QAM “Q” axis. Once again, due to thediffering signal intensity levels (except here 4-bit transmitter 1signal is at the higher power), the QAM signals from the two channelscan be solved for (e.g. determined) by high/low-order bit-separating(e.g. improved) software operating at the invention's improved QAMreceiver(s).

FIG. 9 shows a diagram of how the invention's techniques of crosstalkcorrection and using two cable modem QAM transmitters on one channel maybe combined to improve the upstream data carrying capability of a CATVcable system.

In this scheme, as previously discussed, CATV data is being transmittedfrom cable plant or head (800) to various local neighborhood CATV cables(802) and households with cable modems (804)-(816). Some portions of theneighborhood CATV cable may contain RF amplifiers (818). The head end ofthe CATV system may additionally use Cable Modem Termination Systems(CMTS) (820) to manage the system, and may transmit over optical fiber(822) to various DOCSIS receivers which may, for example, be located inoptical fiber nodes (824).

Under the prior art DOCSIS scheme (830), the cable modems in the sevenhouseholds (804)-(816) might, for example, be forced to transmitupstream data, on a one channel per modem per time basis, on variousupstream channels (832), (834), (836), (838) separated by comparativelywide guard bands (840). The comparatively larger amount of CATV spectrumallocated to CATV downstream data channels is shown as (842). As issymbolized by mismatch between the seven cable modems and the fourupstream data channels, there can be an upstream data bottleneck.

By contrast, according to the invention, this upstream bottleneck can bereduced by using an improved DOCSIS receiver (824) capable of reducingcrosstalk and/or capable of distinguishing between two different cablemodem transmitters transmitting at the same time, same frequency, andsame SCDMA code. Here, for example, node (822) or CMTS (820) maybroadcast a command, such as various MAC level offset frequency adjustparameter commands, to cable modems (804)-(816) instructing them to nowtransmit on alternate more closely spaced DOCSIS upstream channels(850). Because the guard bands are reduced or absent, now more upstreamDOCSIS channels are possible (here 6 channels, rather than the previous4 channels).

Additionally, for sill higher upstream transmission efficiency, the CMTS(820) or node (824) may additionally transmit other DOCSIS MAC cablemodem commands, such as a command to cable modems (804) and (806) totransmit on the same channel and at the same time and SCSMA code, butwhere cable modem (806) is now transmitting at half power levels. This2× loaded DOCSIS upstream channel can be seen as channel (852). Thesystem may similarly instruct cable modems (810) and (812) to do thesame thing, producing two transmitters per channel loaded DOCSISupstream channels (854). Cable modem (816) may also now be instructed totransmit on a new DOCSIS upstream channel (856) which is now madepossible due to the increased upstream spectrum that was made availableas a result of the reduction of the guard bands (840).

1. A method of improving the upstream data transmission rate of a DOCSISCATV cable system, said method comprising: providing an improved DOCSISreceiver capable of correcting for cross-talk between adjacent DOCSISchannels; directing a plurality DOCSIS cable modems to transmit upstreamusing a plurality of adjacent channel frequencies selected with reducedor absent guard bands; receiving said plurality of adjacent channelfrequencies; and using said improved DOCSIS receiver to correct forcross talk between said adjacent DOCSIS channels.
 2. The method of claim1, wherein said plurality of DOCSIS cable modems transmit a plurality ofadjacent channel frequencies, each said frequency separated by a guardband bandwidth of less than or equal to one half α*Fs, where Fs is thebandwidth of each said channels, and α is the roll-off factor of thebandwidth of each of said channels.
 3. The method of claim 1, wherein NDOCSIS cable modems transmit N adjacent channel frequencies withbandwidth Fs, and wherein the bandwidth occupied by said N adjacentchannels, including the roll off at the two extreme boundaries of said Nadjacent channels is (N+α)*Fs, where Fs is the bandwidth of each of saidchannels, and α is the roll-off factor of the bandwidth of the twochannels at the two extreme boundaries of said N adjacent channels. 4.The method of claim 1, wherein said DOCSIS receiver corrects forcross-talk between adjacent DOCSIS channels using adaptive signalseparation or signal equalization methods.
 5. The method of claim 1,wherein said DOCSIS receiver operates by partitioning the incomingclosely spaced DOCSIS channel signals through a polyphase filter bank,and further processing the output from said polyphase filter bank usingFast Fourier Transform methods.
 6. The method of claim 5, wherein saidpolyphase filter bank partitioning and said Fast Fourier Transformmethods are implemented using at least one Digital Signal Processor. 7.The method of claim 1, wherein directing said plurality of DOCSIS modemsto transmit upstream using a plurality of channel frequencies selectedwith reduced or absent guard bands is done by transmitting a pluralityof DOCSIS Offset Frequency Adjust parameters to said plurality of DOCSIScable modems.
 8. The method of claim 1, further improving the upstreamdata transmission rate of said DOCSIS CATV cable system by additionallyperforming the steps of: providing an improved DOCSIS receiver systemcapable of frequency synchronizing and transmitter level adjusting aplurality DOCSIS cable modems; directing at least a first DOCSIS cablemodem to upstream transmit its first signals at a first range of signalintensity levels at a given time and given frequency; directing at leasta second DOCSIS cable modem to transmit its second signals at a secondrange of signal intensity levels at the same time, same frequency, andsame SCDMA code as said first DOCSIS cable modem; receiving combinedfirst and second DOCSIS cable modem signals at said improved DOCSISreceiver system; and separating said first signals from said secondsignals.
 9. The method of claim 8, wherein directing said first DOCSIScable modem to upstream transmit its first signals at a first range ofsignal intensity levels and directing said second DOCSIS cable modem toupstream transmit its second signals at a second range of signalintensity levels is done by carrier, clock, and phase synchronizing saidfirst and second cable modems, and transmitting at least one DOCSISPower Level Adjust or Transmit Equalization Set parameters to said firstand second cable modems.
 10. The method of claim 8, wherein said secondrange of signal intensity levels has a value that is half that of saidfirst range of signal intensity levels.
 11. The method of claim 8,wherein said first DOCSIS cable modem transmits using the same type ofQAM protocol as said second DOCSIS cable modem, or wherein said firstDOCSIS cable modem transmits using a type of QAM protocol that isdifferent from said second DOCSIS cable modem.
 12. A method of improvingthe upstream data transmission rate of said DOCSIS CATV cable system,said method comprising: providing an improved DOCSIS receiver systemcapable of frequency synchronizing and transmitter level adjusting aplurality DOCSIS cable modems; directing at least a first DOCSIS cablemodem to upstream transmit its first signals at a first range of signalintensity levels at a given time and given frequency; directing at leasta second DOCSIS cable modem to transmit its second signals at a secondrange of signal intensity levels at the same time, same frequency, andsame SCDMA code as said first DOCSIS cable modem; receiving combinedfirst and second DOCSIS cable modem signals at said improved DOCSISreceiver system; and using said improved DOCSIS receiver system toseparate said first signals from said second signals.
 13. The method ofclaim 12, wherein directing said first DOCSIS cable modem to upstreamtransmit its first signals at a first range of signal intensity levelsand directing said second DOCSIS cable modem to upstream transmit itssecond signals at a second range of signal intensity levels is done bycarrier, clock, and phase synchronizing said first and second cablemodems, and transmitting at least one DOCSIS Power Level Adjust orTransmit Equalization Set parameters to said first and second cablemodems.
 14. The method of claim 12, wherein said second range of signalintensity levels has a value that is half that of said first range ofsignal intensity levels.
 15. The method of claim 12, wherein said firstDOCSIS cable modem transmits using the same type of QAM protocol as saidsecond DOCSIS cable modem, or wherein said first DOCSIS cable modemtransmits using a type of QAM protocol that is different from saidsecond DOCSIS cable modem.
 16. The method of claim 12, further improvingthe upstream data transmission rate of said DOCSIS CATV cable system byadditionally performing the steps of: providing an improved DOCSISreceiver capable of correcting for cross-talk between adjacent DOCSISchannels; directing a plurality DOCSIS cable modems to transmit upstreamusing a plurality of channel frequencies selected with reduced or absentguard bands; receiving said plurality of adjacent channel frequencies;and using said improved DOCSIS receiver to correct for cross talkbetween said adjacent DOCSIS channels.
 17. The method of claim 16,wherein said plurality of DOCSIS cable modems transmit a plurality ofadjacent channel frequencies, each said frequency separated by a guardband bandwidth of less than or equal to one half α*Fs, where Fs is thebandwidth of each said channels, and a is the roll-off factor of thebandwidth of each of said channels.
 18. The method of claim 16, whereinN DOCSIS cable modems transmit N adjacent channel frequencies withbandwidth Fs, and wherein the bandwidth occupied by said N adjacentchannels, including the roll off at the two extreme boundaries of said Nadjacent channels is (N+α)*Fs, where Fs is the bandwidth of each of saidchannels, and α is the roll-off factor of the bandwidth of the twochannels at the two extreme boundaries of said N adjacent channels. 19.The method of claim 16, wherein said DOCSIS receiver corrects forcross-talk between adjacent DOCSIS channels using adaptive signalseparation or signal equalization methods.
 20. The method of claim 16,wherein said DOCSIS receiver operates by partitioning the incomingclosely spaced DOCSIS channel signals through a polyphase filter bank,and further processing the output from said polyphase filter bank usingFast Fourier Transform methods. 21-22. (canceled)